NCS is an effective antitumor protein awaiting Phase I clinical studies. The main objects of the proposed research are a) to chemically or enzymatically modify the protein, b) to look for an active fragment in the molecule, c) to study the effects on leukemic cells, and d) to clarify the mechanism of action of NCS. We have evidence to show that the two free amino functions in NCS can be chemically modified without appreciable change in the biological activity. By taking advantage of this property, H3 or I125-labeled NCS and an immobilized NCS have been prepared. Both the derivatives are active in the inhibition of growth of CCRF-CEM leukemic cells in vitro. Since agarose immobilized NCS can inhibit leukemic cell growth, it is assumed that NCS may exert its influence on the cell by binding to or reacting with a receptor on the cell surface. Evidence will be sought on a) kinetics of binding to leukemic cells, b) changes in properties of cells in the presence of immobilized or radioactive NCS, and c) effect on inhibition of DNA synthesis in the cell. We have evidence to suggest that the NH2-terminal 20-amino acid peptide and the -COOH terminal heptapeptide can be cleaved without affecting the biological activity of NCS. Experiments will be directed towards isolation of the active core. Also, efforts will be made to chemically modify the arginine and cystine residues. BIBLIOGRAPHIC REFERENCE: Samy, T.S.A. Preparation of a radiolabeled and biologically active neocarzinostatin. Proc. Am. Assoc. Cancer Res., 16, 116 (1975).